1. Field of the Invention
The present invention relates to a dielectric ceramic composition used as a dielectric layer of a for example multilayer ceramic capacitor etc., an electronic device using this dielectric ceramic composition as a dielectric layer, and a method for producing the same.
2. Description of the Related Art
A multilayer ceramic capacitor, one example of an electronic device, is formed by printing a conductive paste on to a green sheet comprised of a predetermined dielectric ceramic composition, stacking a plurality of such green sheets printed with the ceramic paste, and firing the green sheets and internal electrodes together.
Conventional dielectric ceramic compositions had the property of being reduced and being made semiconductive by firing in a low oxygen partial pressure neutral or reducing atmosphere. Therefore, when producing a multilayer ceramic capacitor, one was forced to fire in a high oxygen partial pressure oxidizing atmosphere. Along with this, it was necessary to use, as the material of the internal electrodes fired simultaneously with the dielectric ceramic composition, an expensive precious metal which would not melt at the temperature of sintering of the dielectric ceramic composition and would not be oxidized even if fired under an oxidizing atmosphere (for example, palladium, platinum, etc.) This proved to be a major obstacle in reducing the price of the multilayer ceramic capacitors produced.
In the face of this, for use of an inexpensive base metal (for example, nickel, copper, etc.) as the material of the internal electrodes, it is necessary to develop a dielectric ceramic composition which does not become semiconductive even if fired under a neutral or reducing atmosphere at a low temperature, that is, is superior in resistance to reduction, and has a sufficient dielectric constant and superior dielectric characteristics (for example, small rate of change of temperature coefficient of capacitance) after firing.
In the past, various proposals have been made for dielectric ceramic compositions enabling use of a base metal for the material of the internal electrodes.
For example, Japanese Unexamined Patent Publication (Kokai) No. 1988-224108 discloses a dielectric ceramic composition containing as a main component a dielectric oxide of a composition expressed by (Sr1−xCax)m(Ti1−yZry)O3 (where, 0.30≦x≦0.50, 0.03≦y≦0.20, 0.95≦m≦1.08) and containing as subcomponents, with respect to 100 parts by weight of this main component, 0.01 to 2.00 parts by weight of Mn converted to MnO2 and 0.10 to 4.00 parts by weight of SiO2.
Further, Japanese Unexamined Patent Publication (Kokai) No. 1988-224109 discloses a dielectric ceramic composition containing, with respect to the above main component, 0.01 to 1.00 part by weight of Zn in addition to the Mn and SiO2.
Further, Japanese Unexamined Patent Publication (Kokai) No. 1992-206109 discloses a dielectric ceramic composition containing as a main component a dielectric oxide of a composition expressed by (Sr1−xCax)m(Ti1−yZry)O3 (where, 0.30≦x≦0.50, 0.00≦y≦0.20, 0.95≦m≦1.08) and having a particle size of the powder in the range of 0.1 to 1.0 μm.
Further, Japanese Examined Patent Publication (Kokoku) No. 1987-24388 discloses a dielectric ceramic composition containing as a main component a dielectric oxide of a composition expressed by (MeO)kTiO2 (where, Me is a metal selected from Sr, Ca, and Sr+Ca and k is 1.00 to 1.04) and containing as a glass component, with respect to 100 parts by weight of this main component, 0.2 to 10.0 parts by weight of Li2O,M (where, M is at least one type of metal oxide selected from BaO, CaO, and SrO) and SiO2 used in a predetermined molar ratio.
Further, Japanese Patent No. 2508359 (Japanese Unexamined Patent Publication (Kokai) No. 1992-14704) discloses a dielectric ceramic composition containing as a main component a dielectric oxide of a composition expressed by (Sr1−xCax)m(Ti1−yZry)O3 (where, 0.35≦x≦0.41, 0<y≦0.1, m=1.00) and containing as subcomponents, with respect to 100 parts by weight of this main component, 0 part by weight to 3.0 parts by weight, exclusive, of SiO2.
Further, Japanese Examined Patent Publication (Kokoku) No. 1993-18201 discloses a dielectric ceramic composition containing as a main component a dielectric oxide of a composition expressed by (Sr1−xCax)m(Ti1−yZry)O3 (where, 0<x<1.0, 0.005≦y≦0.10, 1.00≦m≦1.04) and containing as subcomponents, with respect to 100 parts by weight of this main component, specific ranges of Li2O, SiO2, and MO (where, MO is at least one metal oxide selected from BaO, MgO, ZnO, SrO, and Ca).
Further, Japanese Examined Patent Publication (Kokoku) No. 1996-24006 (Japanese Unexamined Patent Publication (Kokai) No. 1988-224106) discloses a dielectric ceramic composition containing as a main component a dielectric oxide of a composition expressed by (Sr1−xCax)m(Ti1−yZry)O3 (where, 0.30≦x≦0.50, 0.03≦y≦0.20, 0.95≦m≦1.08) and containing as subcomponents, with respect to 100 parts by weight of this main component, 0.01 to 2.00 parts by weight of Mn converted to MnO2, 0.10 to 4.00 parts by weight of SiO2, and 0.01 to 1.00 part by weight of MgO.
The dielectric ceramic compositions of these publications, however, all suffered from the problems of a short accelerated life of the insulation resistance after firing and of a lower reliability of the multilayer ceramic capacitor obtained when producing a multilayer ceramic capacitor using the dielectric ceramic compositions and having internal electrodes of nickel or another base metal.